If changing to sreed fixes it then it's probably your card like you suspect as ATI are known to have issues with 3d apps in OpenGL. It shouldn't have any effect on your rendering quality though. Other things to check for when you get problems after long renders is overheating and insufficient power. Renders make your CPU's power consumption go through the roof and they also cause a lot of heat. If your PSU can't supply that, or your system isnt able to deal with the heat, it will often crash.

Anyways as for a new system, a decent mid range one would be an Intel Q6700 + 8GB RAM + 8800GT + whatever.

A decent high end one would be an Intel i7 920 + 12GB RAM + nVIDIA 260GTX + whatever.

Avoid AMD, avoid ATI, avoid Seagate 1TB+ hard drives. Get a good high quality PSU from Corsair or Antec. You will almost certainly want to overclock your CPU to a minimum of 3GHz (i7s can go to 3.6GHz no problem) so get a nice CPU cooler for it. Win7 64 is the OS you want to eventually buy as it's everything that's good about Vista with most of the terrible bits thrown away. Till that goes retail in a few months, just put up with whatever OS you get preinstalled or already have.